Fix dubious use macro for _() / gettext() (#376)

Fix dubious use macro for _() / gettext() (#376)

The macro _() is defined as:

    #define _(a) (gettext(a))

Which has led to a lot of functions omitting the ()s, e.g.

    mutt_message _("string");

This only compiles because of a side-effect of the macro.

In addition, some translatable strings didn't have a _()
wrapper, so it was added